The present invention relates generally to fiber-optic communication systems. More specifically, the present invention relates to multiple-level modulation in a wavelength-division multiplexing (WDM) optical systems.
Multiple-level modulation schemes have been used in optical systems to a limited extent. For example, one optical system has been considered where pulse-position modulation (PPM) was implemented at 2, 4, 8 and 16 levels, at bit rates of 10 Mb/s and 30 Mb/s over non-directive, indoor, infrared channels. See Audel, M. D., et al., xe2x80x9cPerformance of Pulse-Position Modulation on Measured Non-Directed Indoor Infrared Channels,xe2x80x9d IEEE Transactions on Communications, Vol. 44, No. 6, June 1996. This type of indoor infrared link has a relatively short distance and relatively low noise; multiple path interference is a predominate impairment.
Multiple-level modulation schemes have also been used in microwave systems where signals are converted to optical wavelengths for intermediate transmissions. In one such system, microwave frequency signals are converted to optical subcarriers using multiple-level, phase-shift key (PSK) modulation. See Walker, S. D. et al., xe2x80x9cMultivariate Optimisation Criteria for Microwave Subcarrier PSK Optical Transmission Systems,xe2x80x9d IEE Colloquium on xe2x80x98Microwave Optoelectronicsxe2x80x99 (Digest No. 139), pp. 11/1-4, 1990. Again, this system transmits the optical signals over a link having a relatively short distance and a relatively low noise.
These types of systems, however, cannot be used effectively over links having different characteristics, such as highly non-linear, non-repeatered links over relatively long distances. Systems requiring the preservation of optical carrier phase over the link cannot be used over links having relatively long distances. Such systems include for example coherent systems, systems using multiple-level quadrature amplitude modulation (QAM), and systems using multiple-level phase shift key (PSK) on an optical carrier.
Multiple-level intensity signaling systems also suffer insurmountable problems associated with their respective xe2x80x9csymbol-to-noisexe2x80x9d ratio over nonlinear links. In such links, the noise accumulates over the length of the respective links. The signal power, however, cannot be increased beyond a certain point due to the increase in nonlinear distortion of the optical signals. Consequently, beyond this point where the signal power cannot be increased without increasing the noise, the noise is prohibitively large so that the symbols cannot be effectively detected and the data cannot be recovered effectively. In other words, symbol-to-noise ratio reaches a peak for a certain signal power beyond which the noise increases and the symbol-to-noise ratio decreases as the signal power increases.
Embodiments of the present invention address the shortcomings of the known applications of multiple-level modulation over nonlinear links having relatively long distances. Optical signals modulated with data are transmitted into a fiber link having a property of including nonlinear distortion of the optical signals as a function of signal power of the optical signals and distance traversed in the fiber link. The data is formatted with multiple-level signaling having a preselected M value to produce formatted data signals. The formatted data signals are multiplexed to produce wavelength-division multiplex (WDM) channels each carrying optical signals based on the formatted data signals. The preselected M value is based on signal jitter and maximum signal power.